The present invention relates to a novel secondary battery using a non-aqueous electrolytic solution, a process for producing the same and an electrical appliance using the same, and particularly to a lithium secondary battery having distinguished charging/discharging characteristics such as a higher voltage, a higher energy density, a higher charging/discharging capacity and a longer cycle life as well as a higher safety, a process for producing the same and uses of the same.
With increasing needs for use of portable appliances in the field of electronic appliances, miniaturization and weight reduction of appliances are underway, so that development of higher energy density batteries, particularly secondary batteries, has been keenly desired. A lithium secondary battery is one of candidates for secondary batteries satisfying such requirements. Lithium secondary battery has a high voltage and a high energy density and also a light weight, as compared with a nickel-cadmium battery, a lead storage battery and a nickel-hydrogen battery. However, a lithium secondary battery using lithium metal as a negative electrode-active material has problems of short battery life and poor safety because lithium tends to deposit on the negative electrode surface as dendrites, resulting in occurrences of an internal short-circuit to the positive electrode and inactivation toward the electrolytic solution. To avoid risks of using lithium metal, lithium secondary batteries using lithium alloys such as Lixe2x80x94Pb, Lixe2x80x94Al, etc. as negative electrode-active materials have been developed. However, even these lithium secondary batteries still suffer from problems of dendrite deposition and pulverization, so that no satisfactory battery life has been obtained yet. On the other hand, a lithium secondary battery using graphite as a negative electrode-active material has been developed and is now in practical use, where the graphite can store/discharge lithium ions by reactions of diffusing lithium ions into between the c planes of graphite or releasing therefrom, while it is more stable than the chemically active metallic lithium and is free from deposition of lithium dendrites, resulting in prolonged cycle life and increased safety.
In case of using graphite as a negative electrode-active material, the discharge capacity is 370 Ah/kg at most. To increase the capacity of the lithium secondary battery, it is indispensable to use negative electrode-active materials of higher capacity. The negative electrode-active materials of higher capacity include Al, Pb, etc., i.e. elements capable of forming intermetallic compounds with Li, but suffer from a rapid cycle deterioration when used alone or in combination with electroconductive particles as a negative electrode-active material, and thus have not been practically used as a negative electrode-active material. Composite oxides of Sn or Si have been regarded as negative electrode materials of higher capacity, as disclosed, for example, in JP-A-9-213329 and JP-A-8-236158. However, these composite oxides have a high initial capacity, but suffer from a high irreversible capacity, a low Coulomb efficiency and a short cycle life, and thus have not been used as negative electrode-active materials for the lithium secondary battery. So far proposed methods for improving the cycle life of elements capable of forming alloys with an alkali metal, on the other hand, include, for example, a method for coating particles of an element capable of forming an alloy with an alkali metal with a carbonaceous material (JP-A-6-279112) and a method for coating fine particles of Al, Si or the like with a carbonaceous material (JP-A-10-3920). However, it has been found that these carbon-coated materials undergo oxidation of metallic element in the charging/discharging process, resulting in lowered electro-conductivity and considerably lowered charging/discharging characteristics.
An object of the present invention is to provide a lithium, secondary battery with improved deterioration of the characteristics, a higher capacity and a distinguished charging/discharging cycle characteristic, a process for producing the same and electric appliances using the same.
The present invention provides a lithium secondary battery, which comprises a positive electrode, a negative electrode containing a lithium ion-storable/dischareable negative electrode-active material and a lithium ion conductive, non-aqueous electrolytic solution or polymer electrolyte, characterized in that the negative electrode-active material comprises oxide particles containing at least one element selected from Si, Sn, Ge, Al, Zn, Bi and Mg, and particles of carbonaceous material, the oxide particles being embedded in the particles of carbonaceous material.
The present invention also provides a lithium secondary battery, which comprises a positive electrode, a negative electrode containing a lithium ion-storable/dischargeable negative electrode-active material and a lithium ion-conductive, non-aqueous electrolytic solution or polymer electrolyte, characterized in that the negative electrode-active material comprises a composite powder containing particles of carbonaceous material and lithium ion-interstitially diffusible/releasable particles selected from at least one of metal particles and metal oxide. particles capable of enhancing a lithium ion interstitial diffusibility/releasability to/from the carbonaceous material between the positive electrode and the negative electrode when charged/discharged, at least 50% by weight of the interstitially diffusible/releasable particles being embedded in the particles of carbonaceous material and the interstitially diffusible/releasable particles having an average particle size of not more than 5 xcexcm and particle sizes of not more than 10 xcexcm for at least 90% by weight thereof.
The present invention further provides processes for producing these lithium secondary batteries, negative electrode materials for lithium secondary batteries, and use of these lithium secondary batteries.